Molecular breeding and industrialization of forage sweet sorghum

Abstract

Sorghum (Sorghum bicolor (L.) Moench) is the world's fifth major crop and a C4 model plant. Sweet sorghum is a variant of grain sorghum with implications in biofuel industry and stress biology because of it remarkable traits including high biomass production, high concentration of soluble sugars in its juicy stem, and strong abiotic tolerance. In recent years, there is a new demand to develop sweet sorghum into a forage crop, which requires the tailor-made modification of its major biological characteristics. Compared to silage maize, sweet sorghum possesses higher levels of directly fermentable reducing sugars and the ability to accumulate high biomass under low-input production systems. In addition, it is tolerant to drought and more efficient in utilization of solar radiation and nitrogen-based fertilizers than maize on marginal lands which are not optimal for food production. These traits collectively make sweet sorghum more attractive than silage maize with huge potential as a forage crop. Yet, due to the relatively short history of sweet sorghum breeding, the development of new varieties adapting to various phenological requirements is restricted, and the rapid deployment of sweet sorghum for forage production is limited. Here we summarize the current status of forage sweet sorghum breeding in China. A suit of sweet sorghum traits, such as plant architecture (leave, and stem), flowering time and maturity, leaf senescence and accumulation of juice and sugar as well as drought and salt tolerance are identified, and recent progression in dissecting their genetic basis reviewed. It is suggested that newly developed cutting-edge technologies such as genome editing and whole-genome selection are needed to harness the genetic improvement of sweet sorghum. Feeding experiments with improved varieties showed that sweet sorghum is an important component of the livestock diet for increasing the palatability and nutrient conversion efficiency. The production-scale demonstration project, forming a production pipeline from sweet sorghum hybrid production, cultivation, distillation and feeding of livestock and cycle of manures, showed that forage sweet sorghum based bio-industry sets a model for circular economy in rural areas. Future forage sweet sorghum breeding and approaches to develop its bio-industry are discussed.